High temperature austenitic alloy



Wyomissing, Pa., assignors to The Carpenter Steel Comv pany, Reading,Pa., a corporation of New Jersey No Drawing. Filed Mar. 18, 1963, Ser.No. 266,009

8 Claims. (Cl. 75-171) This invention relates to an austenitic nickelbase alloy which is age hardenable and more particularly to such analloy having high strength at elevated temperatures.

Hitherto, for use where high strength at elevated temperature had beenrequired austenitic nickel base alloys were provided having acomposition balanced so as to respond to an aging or precipitationhardening treatment. More particularly such alloys were balanced so thatupon aging they underwent a vprecipitation hardening and strengtheningmechanism such that a precipitate was formed having a lattice structurethe same as that of the alloy matrix in which it is formed and alsohaving a lattice parameter which is close to but yet constitutes amisfit with the lattice parameter of the matrix.

The present invention is'particularly concerned with improving thosenickel base austenitic precipitation hardening alloys which hardenduring aging by a mechanism involving the formation of a face centeredcubic re States ,a tent O precipitate known as"gamma prime. Theusefulness of I such alloys has-been limited by the fact that the gammaprime precipitate transformed during use at elevated temperature after arelatively'short time so that the composition lost its strength. Thistransformation results from the fact that the gamma prime, face centeredcubic structure formed in the original aging treatment underwent afurther transformation to a hexagonal close packed structure, known aseta phase. The extent of the transformation depends upon the duration ofthe exposure to temperatures between about 1100 F. to 1700 F. and

the degree of stress to which the part formed of the alloy is subjected,the transformation proceeding more rapidly as the temperature or thestress is increased.

It is therefore a principal object of the present invention toprovide anage hardening austenitic nickel base alloy characterized by highstrength, high stress rupture ductility and longer useful life understress at elevated temperatures up to about 1400 to 1600 F. by means ofthe formation of a gamma prime precipitate with an optimum'latticeparameter relative to the matrix and which has enhanced stabilityagainst transformation to the undesired eta phase.

The alloy of the present invention is particularly well suited for usein forming such parts as wheels and rings for use in jet engines. Thus,the alloy of the present invention is not only characterized by highstrength and stress rupture ductility at elevated tem'peratures'but alsois characterized'by good resistance to oxidation and corrosion at suchtemperatures.

A more specific object of the present invention is to provide such analloy which may be readily melted and cast into ingots that may beeconomically hot worked and formed into parts.

We have discovered that the foregoing objects can be achieved with anaustenitic alloy having the following analysis in percent by weightwithin the tolerances of good commercial melting practices:

p CC

Vanadium Up to 1.0%. Titanium 2.75% to 3.75%. Aluminum .75% to 2.0%.Boron .003% to .025%.

temperatures as high as 1600" F. is to be obtained suit able for suchuse as in the fabrication of jet engine parts.

In this alloy, the maximum carbon content must not w exceed about .10%and is preferably maintained at a muchlower level below about .07%maximum. Vanadium may be included in this composition in amounts rangingup i when desired to tie up the carbon and reduce to about 1% theformation of a titanium carbide. Manganese may be present in amounts upto about 2% but ispreferably limited to no more than .25%.

When large commercial ingots are melted, silicon, when present in morethan very small amounts, results in the formation of anickel-titanium-silicide segregate, believed to be Ni Ti Si identifiedas G phase in forged billets. For many purposes such segregate areas inthe billets are considered unsound and, to avoid their-formation,silicon is limited to no more than about 25%.

Small additions of boron work to improve the rupture strength of ouralloy at eleyated temperatures. For this purpose, from about .003% to.025% boron provides good results. When present in amounts above .025%,boron tends to form borides, particularly with nickel, and this has adetrimental effect upon the forgeability and mechanical properties ofthe alloy. Preferably, lower amounts'below .0l3% boron are utilized toprovide optimum results.

Chromium from about 12% to 20% provides the required degree of oxidationand corrosion resistance. Because chromium does not enhance the hightemperature strength of our alloy and because it tends to form anembrittling carbide, we preferably use the smaller amounts of chromium,from about 12.5% to 15%.

Nickel also provides oxidation and corrosion resistance,

but more importantly nickel takes part in the aging mechanism by 'whichour alloy is strengthened. A minimum of about 35% nickel is required forthis purpose. When nickel ispresent in amounts above about insuflicientadvantage is derived therefrom to justify the addi- Carbon -5. Up to.10% maximum; Manganese Up to 2%.

Silicon Up to 1% maximum. Chromiumfl- 12% to 20%.

Nickel 35% to 55%. Molybdenum 3% to 7%.

Cobalt 1.5% to 7%.

"the hot workability of the composition.

tional cost. Best results are achieved when nickel is present in amountsranging from about 40% to 45%;

Molybdenum is included because it'improves the high temperature strengthof the alloy. For this purpose,

molybdenum is included in an amount ranging from about Titanium largelycontributes to the rupture strength and the tensile strength of thealloy and for this purpose about 2.75% to 3.75% titanium is utilized.When titanium is present in an amount less than about 2.75%,

both the high room temperature tensile strength and the! high rupturestrength at elevated temperatures characteristic of our alloy are notattainable. Best results are achieved with a minimum of about 3%titanium. Larger amounts of titanium than about 3.75% tend to impairTitanium, in the absence of at least about .75% nickel during aging toprovide a meta stable face centered cubic phase which in turn transformsto the nudesired hexagonal close-packed eta phase at the elevatedtemperature at which parts formed from the alloy are intended to beused.

When aluminum is present in ouralloy in amounts Patented May 11, 1965,

aluminum reacts with varying from about .75% to 2.0%, the more stablegamma prime phase is formed on aging identified as Ni (Ti, Al), mostconsistent results being attained when aluminum is present in an amountvarying from about 1% to 1.5%. Larger amounts of aluminum not onlydetract from the high temperature strength of our alloy but also workwith the titanium to make hot working of the alloy increasinglydifiicult.

With the elements balanced as already described hereinabove and withcritically controlled additions of cobalt as now to be described, ouralloy has outstanding strength at high temperature. In fact, though ourpreferred alloy contains as much as about 25% to 35% iron, neverthelessits strength at elevated temperatures up to 1600 F. is essentiallyequivalent to the high temperature strength at correspondingtemperatures of much more expensive precipitation hardenable nickel basealloys containing as little as 0.5% iron.

Our experiments have shown that the unique high temperature strength ofour alloy results from a definite but small degree of misfit between thelattice parameter of the matrix and the lattice parameter of the gammaprime phase formed by aging, that is, the face centered cubic Ni (Al,Ti). By maintaining the cobalt content between about 1.5% and 7%, thelattice parameters are kept so close to each other that the percentmisfit characteristic of our alloy is uniquely small. When smaller orlarger amounts of cobalt than the amount specified is present, then therequired low percent misfit between the lattice parameters and the hightemperature strength characteristic of our alloy are not attained. Bestresults are achieved when the cobalt content ranges from about 3% to 5%.

Thus, the alloy which we have produced with preferred results has thefollowing composition in percent by weight within the tolerances of goodcommercial melting practice:

Carbon Up to .07% maximum. Manganese Up to .25% maximum. Silicon Up to.25 maximum. Chromium 12.5% to 15%. Nickel 40% to 45%. Molybdenum 4% to6%.

Cobalt 3% to 5%.

Titanium 3%.to 3.5%. Aluminum 1% to 1.5%.

Boron .003% to .013%. Vanadium Up to 1%.

the balance consisting essentially of iron except for incidentalimpurities.

Our alloy is readily prepared and worked in accordance with goodstandard commercial practice. Our alloy may be prepared with a highdegree of purity in commercial quantities using conventionalvacuum-induction or consumable-electrode melting techniques. No specialheat treatment is required. Solution treatment is about 180 F to 2050 F.followed by a single or double aging or precipitation hardeningtreatment in the range of about 1200 F. to 1600 F. provides goodresults.

As a specific example of our alloy, an ingot was melted and castcontaining, in percent by weight:

Boron 0.009

and the balance all iron except for incidental impurities. The ingot washot worked into inch bars, then solution treated and aged. In thisinstance, the solution treatment was carried out at 2000 F. for twohours followed by water quenching. Aging was carried out by holding thesolution treated alloy for two'hours at 1525 F., air cooling and thenholding for twenty-four hours at 1400 F. and air cooling,

The aged bars were machined to form standard combination smooth-notchstress rupture specimens having a smooth section 0.178 inch in diameterand a gauge length of 0.712 inch merging with a thicker section 0.250inch in diameter in which a circular notch was formed :having a 0.178inch diameter at the root of the notch, a root radius of 0.005 inch anda notch angle of 60", thereby providing a stress concentration factor of3.8. At 1400 F., a stress load of 39,000 psi. was sustained for 573hours before rupture with a 10.5% elongation and 23.3% reduction inarea. The failure occurred in the smooth portion of the test specimensand not at the notch, indicating that the alloy has good stress ruptureductility.

Thus, highly beneficial results are attained when our alloy is preparedhaving the following composition in percent by weight within thetolerances of good commercial melting practice: I

Carbon Up to'.06% maximum. Manganese Up to .25% maximum. Silicon Up to.25 maximum. Chromium 12.5% to 13.5%. Nickel 42.5% to 43.5%. Molybdenum5.0% to 6.0%.

Cobalt 3.5% to 4.5%. Titanium 2.8% to 3.3%. Aluminum 1.0% to 1.4%.

Boron .008% to .01%.

the balance consisting essentially of iron except for incidentalimpurities.

On the other hand, tests carried out with similarly prepared specimenshaving essentially the same analysis but not containing cobalt werecharacterized by significantly poorer stress rupture strength. Forexample, an alloy containing, in percent by weight:

no cobalt, and the balance iron except for incidental impurities, wasprepared as was described in connection with the alloy of the presentinvention and formed into a standard smooth stress rupture test specimenhaving a 0.178 inch gauge diameter and a 0.712 inch gauge length. Whensubjected to a stress load of 39,000 p.s.i. at 1400 F., rupture occurredin 436 hours with 16.9% elongation and 26.1% reduction in area.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

We claim:

1. An age hardening austenitic nickel base alloy which in its hardenedcondition has good strength and ductility at temperatures up to about1600 F. and which'within I thetolerances of good melting practicesconsists essentially of:

V Percent Carbon Up to .10 Manganese Up to 2 Silicon Up to 1 Chromium 12to Nickel 35 to 55 Molybdenum 3 to 7 Cobalt 1.5 to 7 Vanadium Up to 1Titanium 2.75 to 3.75 Aluminum .75 to 2 Boron .003 to .025

the balance consisting essentially of iron and in which 4. An articlesubjected in use-to severe stress'at elevated temperatures up to about1600 F. and formed of an age hardened austenitic nickel base alloy whichwithin the tolerances of good melting practices consists essentially thealuminum serves to stabilize the gamma prime phase formed by agehardening against transformation to an eta phase at elevated temperatureand under stress, and in which the amount of cobalt providessubstantially minimum misfit between the lattice parameters of thematrix of the alloy and of the gamma prime phase when the aged alloy isexposed to stress at high temperature over a long time.

2. An age hardening austenitic nickel base alloy which in its hardenedcondition has good strength and ductility at temperatures up to about1600 F. and which within the tolerances of good melting practicesconsists essentially of:

Percent Carbon Up to .07 Manganese Up to .25 Silicon Up to .25 Chromium12.5 to 15 Nickel 40 to 45 Molybdenum 4 to 6. Cobalt 3 to 5 Titanium 3to 3.5 Aluminum 1 to 1.5 Boron .003 to .013 Vanadium Up to 1 the balanceconsisting essentially of iron and in which the aluminum serves tostabilize the gamma prime phase formed by age hardening againsttransformation to an eta phase at elevated temperature and under stress,and

in which the amount of cobalt provides substantially minimum misfitbetween the lattice parameters of the matrix of the alloy and of thegamma prime phase when the aged alloy is exposed to stress at hightemperature over a long time.

3. An age hardening austenitic nickel base alloy which in its hardenedcondition has good strength and ductility at temperatures up to about1600 F. and which within the tolerances of good melting practicesconsists essen' tially of:

Carbon Up to .06% maximum. Manganese -e- Up to .25% maximum. Silicon Upto .25% maximum. Chromium 12.5% to 13.5%. Nickel 42.5% to 43.5%.Molybdenum 5.0% to 6.0%. Cobalt 3.5% to 4.5%. Titanium 2.8% to3.3%.Aluminum 1.0% to 1.4%.

. Boron .008/ to 01%.

the balance consisting essentially of iron and in which the aluminumserves to stabilize the gamma prime phase formed by age hardeningagainst transformation to an eta phase at elevated temperature and understress, and in which the amount of cobalt provides substantially minimummisfit between the lattice parameters of the matrix of the alloy and ofthe gamma prime phase when the aged alloy is exposed to stress at hightemperature over a long time.

Percent Carbon Up to .10 Manganese Up to 2 Silicon Up to 1 Chromium 12to 20 Nickel 35 to 55 Molybdenum 3 to 7 Cobalt 1.5 to 7 Vanadium Up to 1Titanium 2.75 to 3.75 Aluminum .75 to 2 Boron .003 to .025

the balance consisting essentially of iron and in which the aluminumserves to stabilize the gamma prime phase formed by age hardeningagainst transformation to an eta phase at elevated temperature and understress, and in which the amount of cobalt provides substantially minimummisfit between the lattice parameters of the matrix of the alloy and ofthe gamma prime phase when the aged alloy is exposed to stress at-hightemperature over a long 5. An article subjected in use to severe stressat elevated temperatures up to about 1600 F. and formed of an agehardened austenitic nickel base alloy which within the aluminum servesto stabilize the gamma prime phase formed by age hardening againsttransformation to an eta phase at elevated temperature and under stress,and in which the amount of cobalt provides substantially minimi'sfitbetween the lattice parameters of the matrix of the alloy and of thegamma prime phase when the aged alloy is, exposed to stress at hightemperature over a long time.

6. An article subjected in use to severe stress at elevated temperaturesup to about 1600 F. and formed of an age hardened austenitic nickel basealloy which within the tolerances of good -melting practices consistsessentially ofi. H v

Carbon Up to .06% maximum. Manganese Up to .25% maximum. Silicon Up to.25% maximum. Chromium 12.5% to 13.5%. Nickel 42.5% to 43.5%. Molybdenum5.0% to 6.0%.

Cobalt 3.5% to 4.5%. Titanium 2.8% to 3.3%. Aluminum 1.0% to 1.4%.

Boron .008% to .01%.

the balance consisting essentially of iron and in which the aluminumserves to stabilize the gamma prime phaseformed by age hardening againsttransformation to an eta phase at elevated temperature and under stress,and in which the amount of cobalt provides substantially minimum misfitbetween the lattice parameters of the matrix of the alloy and of thegamma prime phase when the aged alloy is exposed to stress at hightemperature over a long time.

7. An age hardening austenitic nickel base alloy which in its hardenedcondition has good strength and ductility at temperatures up to about1600 F. and which within the tolerance of good melting practice consistsessentially of:

Percent Carbon Up to .07 Manganese Up to .25 Silicon Up to .25 Chromium12 to 15 Nickel 40 to 45 Molybdenum 4 to 7 Cobalt About 4.5 Titanium 2.8to 3.5 Aluminum l to 1.5 Boron .003 to .01 Vanadium Up to 1 the balanceconsisting essentially of iron.

8. An article subjected in use to severe stress at elevated temperaturesup to about 1600 F. and formed of 8 an age hardened austenitic nickelbase alloy which within tolerances of good melting practices consistsessentially of:

Percent 5 Carbon Up to .07 Manganese Up to .25 Silicon Up to .25Chromium 12 to 15 Nickel 40 to 45 1'0 Molybdenum 4 to 7 Cobalt About 4.5Titanium 2.8 to 3.5 Aluminum 1 to 1.5

Boron .003 to .01 15 Vanadium Up to 1 the balance consisting essentiallyof iron.

References Cited by'the Examiner UNITED STATES PATENTS 3,065,067 11/62Aggen 7s- 124 DAVID L. RECK, Primary Examiner. WINSTON A. DOUGLAS,Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,183,084 May 11, 1965 Gerald B. Heydt et :11.

It is hereby certified that error appears in the above numbered patentreqiiring carrectio'n and that the said Letters Patent should read ascorrectedbelow.

Column 3, lines 57 and 58, for "is about 180 F to 2050 F."

read at about 1800 F. to 2050 F. column 5, line 65, for ".008/ to .01 0"read .008% to .01

Signed and sealed this 21st day of September 1965.

(SEAL) Aug.

ERNEST W. SWIDER EDWARD J. BRENNER Atmsting Officer Commissioner ofPatents

1. AN AGE HARDENING AUSTENITIC NICKEL BASE ALLOY WHICH IN ITS HARDENEDCONDITION HAS GOOD STRENGTH AND DUCTILITY AT TEMPERATURES UP TO ABOUT1600*F. AND WHICH WITHIN THE TOLERANCES OF GOOD MELTING PRACTICESCONSISTS ESSENTIALLY OF: